1. Field of the Invention
The present invention relates to devices used for preventing exposed fluid supply and fluid circulation systems from freezing during cold weather. More particularly, the present invention relates to solar heating systems and devices for preventing heat exchange fluid in such systems from freezing during periods of low insolation when the ambient temperature is below the freezing point of the heat exchange fluid.
2. Discussion of Related Art
Due to the relatively recent large increases in the cost of fossil fuel, solar heating has become increasingly popular. Solar systems are particularly useful for producing hot water for domestic use in place of conventional fossil fuel or electric water heaters. However, problems with such systems occur when the ambient temperature surrounding the solar panel becomes exceedingly low thereby causing the heat exchange fluid in the solar panel to freeze. There are, of course, various means of preventing the heat exchange fluid from freezing. For instance, antifreeze can be used in the fluid itself; however, this solution requires that a separate heat exchanger be used to transfer heat to the domestic hot water. This results in a relatively expensive system. It is preferable to heat the water to be used for domestic purposed directly in the solar panel.
Various solutions have been suggested to prevent domestic water from freezing in solar panels. For example, U.S. Pat. No. 4,119,087 to Cook discloses a system for protecting solar panels from freezing which system includes a temperature sensor disposed to sense the temperature of water in a manifold header of a solar collector. When the temperature drops below a predetermined safe limit, the system circulator pump operates so that the cold water is transferred to the system's storage tank and stored, warm water is transferred to the solar collector. This type of system is inefficient since the energy being stored in the warm water is dissipated to keep the solar panels from freezing.
U.S. Pat. No. 3,986,489 to Schlesinger shows a freeze protection system wherein a thermaster is disclosed in the solar collector and causes a dump valve to open when a temperature drops below a dangerous level. The solar collector is entirely evacuated in this manner so that no freezable liquid remains.
U.S. Pat. No. 3,618,625 to Walters shows a freeze prevention system for a water storage tank such as a cattle trough or the like. Water stored in the tank is prevented from freezing by opening a valve which conducts a restricted outflow of water under control of a temperature sensing element located adjacent the surface of the water where freezing begins. The outflow of water induces inflow of water at a higher temperature from a remote source to maintain the temperature level of the water within the tank above an adjusted value.
U.S. Pat. No. 4,280,478 to Duval et al shows a freeze protection apparatus for solar collectors in which the working fluid contained in the communicating tubes of the solar collector is allowed to drain from both the input and output tubes of the collector when a first predetermined temperature of the working fluid is detected. The collector is allowed to fill through the input and output tubes when a second predetermined temperature is detected.
Various valves have been suggested for permitting a small flow of liquid through a pipe in response to a sensed low ambient temperature in order to prevent the fluid from freezing. U.S. Pat. No. 4,344,450 to Walters and U.S. Pat. No. 4,286,613 to Lacoste show the construction of two such valves. The Lacoste valve uses "Freon 502" in the sensing apparatus of the valve to produce a pressure tending to hold the valve closed when the ambient temperature is at a safe level.
U.S. Pat. No. 1,554,344 to Happanen shows a thermostatic valve for use in a water circulation system to prevent freezing. The valve includes a chamber containing an expansible and contractable fluid which causes the valve to open to allow water to escape when the temperature of the water falls below a safe level.
U.S. Pat. No. 1,971,242 to Wheeler shows a solar water heater having a valve which opens when the temperature of the water and the solar collector increases to a prescribed limit to allow the heated water to enter a storage chamber.
U.S. Pat. No. 4,153,040 to Harrison et al shows a protective system for a solar collector which includes a bulb disposed in the solar collector. The bulb contains a vaporizable antifreeze liquid which expands when the temperature in the collector rises to open a valve which permits overheated water in the solar collector to escape and permits fresh, cool water to enter the solar collector.
U.S. Pat. No. 2,365,650 to Shaw et al shows a water valve structure which is particularly adapted for controlling the flow of cooling water to the condensor of a refrigeration system. The valve is connected to a pressure line such as the refrigerant line of a refrigeration system. The valve opens when pressure in the system builds up thus permitting cooling water to flow through the condensor.